The largest urban sprawl on the planet in medieval times was in fact 10 times bigger than thought, rivalling the size of Greater London.

Carpeted today with vegetation, obscured by a cloak of low-lying cloud and raided by thieves, Angkor in Cambodia once thrived between the 9th and 16th centuries, reaching a peak of many hundreds of thousands of people in the 13th century

Today, in the Proceedings of the National Academy of Sciences, a new map reveals its heart spread over 400 square miles – compared with Greater London’s 600 square miles – and the associated sprawl extended out another several hundred square miles.

There are also features that puzzle archaeologists: an enclosed grid over a square mile containing 100 little mounds to the east of the East Baray, and an arrangement of eight grid-like enclosures of raised embankments between the Angkor-Phimai Road and Prei Vihe’ar/Phnom Dei, each measuring about 250 yards on each side.

Overall, the city is 10 times bigger than previously thought and extends far beyond today’s World Heritage Site, according to Greater Angkor Project deputy director Damian Evans from the University of Sydney.

However, in its heyday it would have only supported around 1,500 people per square mile.

“People usually considered Angkor to be a scatter of discrete temples and if they bothered to consider the size or extent of Angkor at all, they took the central enclosures (for example of Angkor Thom, or the temple enclosures) as being the ‘extent’ of the city,” Mr Evans told The Daily Telegraph.

It only became possible to appreciate how vast the sprawl was after the fall of the Khmer Rouge. To create the map, Mr Evans and his colleagues from Australia, Cambodia, and France worked for about a decade on hand-drawn maps, ground surveys, airborne photography, and ground-sensing radar provided by the US space agency, Nasa.

The radar can show differences in plant growth and moisture content that result from surface variations of less than a metre to reveal the extraordinary “hydraulic city.”

The last major temple to be discovered in the area was in 1914 and the team has now found evidence of another 74, though they are humbler affairs, all arranged on an east-west axis according to religious imperatives.

“The kinds of temples we are discovering now usually consist of little more than small piles of brick rubble, sometimes with some sandstone elements such as door frames or pedestals for statues, and quite often large amounts of Angkorian ceramic shards in the area,” Mr Evans said. “Sometimes there is not even that, even after excavations take place. In these cases we assume that the temple must have been made of wood, of which nothing now remains.”

One single hydraulic system links the entire network and likely provided Angkor’s citizens with a stable water supply despite the unpredictable monsoon season. There has been academic debate about whether or not the hydraulic network was used for intensive rice agriculture – that is, for irrigation. The new study shows the region was indeed carpeted with rice fields, interspersed with settlements of houses, local temples and water storage ponds

“There are inlets and outlets on all the major reservoirs. There are distributor canals – every single water source in the region was intensively and relentlessly exploited – and there was a series of very sophisticated water control devices such as spillways – sometimes massive structures built in stone – which have the capacity to provide an irrigation for rice.”

However, it probably became too big to maintain and there was a warming in the Middle Ages that could have pushed the society to the brink of collapse.

“People have always been interested in the downfall of Angkor,” said Mr Evans. “Our research shows that Angkor was certainly extensive enough and that land use was certainly intensive enough to have impacted profoundly on the regional ecology.”

The region would have faced problems with deforestation, overpopulation, topsoil degradation and erosion.

“In the new maps and in the excavations that we’re doing we can see what looks to be evidence of this – breaches in dykes and barrages, attempts to patch up the system, stratigraphies (deposits of sediments) which suggest chaotic flows of water,” Mr Evans said.

The survey backs the idea that the local Khmer “built themselves out of existence,” Mr Evans added. “It leaves open the possibility that systemic problems in the network could indeed have caused the downfall of the Angkorian state.” At the same time, the Khmer economy moved towards coastal towns.

Although it was the most extensive settlement complex of its time, and was unparalleled until after the industrial revolution, it was not the most populous. “There were cities in China during that time which had a million inhabitants, or more, which I believe is more than Angkor itself would have had,” Mr Evans said.

The size of Greater Angkor, which had between half a million and a million inhabitants, poses huge challenges to conservation efforts.

“The well preserved remains of the urban complex extend far beyond the designated World Heritage zone that surrounds the central temples, highlighting the need to reappraise, in due course, how this remarkable heritage site is to be managed,” Mr Evans added.